Sodium-independent transporter transporting small-sized neutral amino acid, gene thereof and method of analyzing transporter function by constructing fused proteins enabling the specification of the function

ABSTRACT

The invention provides a sodium independent transporter which transports a small-sized neutral amino acid and its analog; its gene; fused proteins; and a method of analyzing the function of the transporter. Further provided is a use for the transporters as described by the invention.

TECHNICAL FIELD

The present invention relates to a protein associated with the sodium-independent transport of a small-sized neutral amino acid and its analogue, a fusion protein thereof, as well as a gene encoding said protein. The invention also relates a method for controlling the cell proliferation or for altering the in vivo pharmacokinetics of a pharmaceutical, toxic substance or exogenous foreign body by modulating an ability to transport a small-sized neutral amino acid and its analogue possessed by a protein associated with the sodium-independent transport of a small-sized neutral amino acid and its analogue, by means of employing said protein, its fusion protein, its specific antibody, or its function-promoting substance or function-suppressing substance, as well as an agent for controlling an ability to transport a small-sized neutral amino acid and its analogue comprising said substances.

Furthermore, the invention relates to a method for analyzing a function of a transporter comprising a step for allowing a transporter protein, whose function can not be identified because of the inability to be transferred to a cell membrane in an exogenous gene expression system, to be transferred to the cell membrane by forming a fusion protein with a protein which promotes the transfer to the cell membrane.

BACKGROUND ART

A cell always requires the uptake of an amino acid as a nutrition, and such a function is exerted by an amino acid transporter which is a membrane protein present in a cell membrane. The amino acid transporter is located in a certain position in each tissue in a multicellular organism and plays an important role in the expression of a specific function of each tissue.

A transport system asc is an amino acid transport system which transports small-sized neutral amino acids such as alanine, serine and cysteine, and was reported originally with regard to an erythrocyte membrane. Thereafter, it was identified also in a cultured cell [Christensen, Physiol. Rev. Vol. 70, page 43, 1990]. The transport system asc is a transporter which is independent of sodium, i.e., whose function requires no sodium ion. Its transport substrate selectivity and transport profile are known to vary somewhat depending on cells and animal species.

While the transport system asc exhibits a high affinity to a transport substrate such as alanine, serine and cysteine, an analogous transport system is known to exist which is a transport system C whose transport substrates are also small-sized neutral amino acids such as alanine, serine and cysteine but which exhibits a lower affinity to the transport substrate [Young et al., Biochem. J. Vol. 154, page 43, 1976; Young et al., Biochem. J. Vol. 162, page 33, 1977]. The transport system C is considered to be a subclass of the transport system asc. A sheep having a genetic defect of the transport system C was identified, and its erythrocyte was found to have a reduced glutathion content, revealing the importance of the cysteine uptake via a cell membrane in the glutathion production [Young et al., Nature, Vol. 254, page 156, 1975].

However, a conventional method involves a difficulty in analyzing the details of the transport of an amino acid or its analogue via the amino acid transport system asc and the in vivo functional roles, and it has been desired to enable a detailed functional analysis by isolating a gene of a neutral amino acid transporter responsible for the function of the amino acid transport system asc.

As small-sized neutral amino acid transporters, ASCT1 and ASCT2 have been cloned [Kanai, Curr. Opin. Cell Biol., Vol. 9, page 565, 1997]. Nevertheless, they are sodium-dependent transporters, and are different in principle from the sodium-independent amino acid transport system asc. Further, a glycine transporter and a proline transporter have also been cloned, however, each transports only glycine or proline in a sodium-dependent manner, unlike to the transport system asc [Amara and Kuhar, Annu. Rev. Neurosci., Vol. 16, page 73, 1993].

The cDNAs of rBAT and 4F2hc, i.e., type II membrane glycoproteins each having only a single membrane-spanning structure which are not the transporters themselves but are considered to be amino acid transporter-activating factors, have been cloned, and are known to activate the uptake of basic amino acids together with neutral amino acids when being expressed in an oocyte of a xenopus [Palacin, J. Exp. Biol., Vol. 196, 123, 1994].

As a transporter which transports neutral amino acids selectively, neutral amino acid transporters corresponding to the transport system L, i.e., LAT1 [Kanai et al., J. Biol., Chem., Vo. 273, page 23629 to 23632, 1998] and LAT2 [Segawa et al., J. Biol. Chem, Vol. 274, page 19745 to 19751, 1999] have been cloned. It was also revealed that the LAT1 and the LAT2 are capable of exerting their functions only when being coexisting with a cofactor 4F2hc which is a single-membrane-spanning type protein. The both are independent of Na⁺, and the LAT1 exhibits an exchange transport activity serving to transport large-sized neutral amino acids such as leucine, isoleucine, valine, phenylalanine, tyrosine, triptophan, methionine and histidine, while the LAT2 exhibits a wide substrate selectivity serving to transport small-sized neutral amino acids such as glycine, alanine, serine, cysteine and threonine in addition to the large-sized neutral amino acids. Nevertheless, their substrate selectivity is also different from that of the amino acid transport system asc.

As proteins analogous to the neutral amino acid transporters LAT1 and LAT2, y⁺LAT1 and y⁺LAT2 having the functions of a transport system y⁺L which transports neutral amino acids and basic amino acids have been cloned [Torrents et al., J. Biol. Chem., Vol. 273, page 32437 to 32445, 1998]. It was also revealed that both of y⁺LAT1 and y⁺LAT2 are capable of exerting their functions only when being coexisting with a cofactor 4F2hc. The y⁺LAT1 and y⁺LAT2 transport mainly glutamine, leucine and isoleucine as neutral amino acids, exhibiting the substrate selectivity different from that of the amino acid transport system asc.

As a transporter requiring the cofactor 4F2hc for exerting its function, xCT which is a protein analogous to the neutral amino acid transporters LAT1 and LAT2 has been cloned [Sato et al., J. Biol. Chem., 274: 11455-11458, 1999]. The xCT transports cystine and glutamic acid, exhibiting the substrate selectivity different from that of the amino acid transport system asc.

Further, as a transporter requiring another cofactor rBAT having a structure analogous to that of 4F2hc, BAT1 which is a protein analogous to the neutral amino acid transporters LAT1 and LAT2 has been cloned [Chairoungdua et al., J. Biol. Chem. 274: 28845-28848, 1999]. The BAT1 transports cystine, neutral amino acids and basic amino acids, exhibiting the substrate selectivity different from that of the amino acid transport system asc.

As described above, a molecular entity of a transporter which functions as a result of the binding to the 4F2hc and the rBAT was characterized and it was revealed that there is a group of the transporters exerting the transporting ability by forming a molecular complex with a type II glycoprotein.

Moreover, as a transporter requiring the cofactor 4F2hc for expressing a function, Asc-1 which is a protein analogous to the neutral amino acid transporters LAT1 and LAT2 has been cloned [Fukasawa et al., Biol. Chem. 275: 9690-9698, 2000]. The Asc-1 transports alanine, serine, cysteine, threonine, glycine and the like selectively, exhibits the substrate selectivity of the amino acid transport system asc, and was proven to be the first isoform of the transporting system asc.

The Asc-1 exhibits a property different from that of a traditional transporting system asc reported with regard to the erythrocyte membrane, since it transports not only L-amino acids but also D-forms of alanine, serine, cysteine and threonine, α-aminoisobutyric acid and β-alanine. Accordingly, it was believed that there is a transporter, other than the Asc-1, corresponding to the traditional transporting system asc.

In an attempt to prepare a fusion protein of a transporter with another protein, b^(0.+)AT was combined with a single-membrane-spanning type cofactor, rBAT, for its transfer to a cell membrane [Pfeiffer et al., Mol. Biol. Cell. 10: 4135-4147, 1999]. Nevertheless, this fusion protein is the one prepared in accordance with an authentic combination of the b^(0.+)AT and the rBAT, and is not intended to allow a transporter protein, which can not be expressed in the cell membrane because of the absence of cofactor, to be expressed forcibly on the cell membrane for the purpose of identifying its functions.

DISCLOSURE OF THE INVENTION

An objective of the invention is to provide a gene of a transporter which transports small-sized neutral amino acids in a sodium-independent manner and exhibits the functions of a traditional transport system asc reported with regard to the erythrocyte membrane as well as a sodium-independent small-sized neutral amino acid transporter which is a polypeptide encoded by said gene. Also by means of producing a fusion protein with this transporter, the invention provides a method for analyzing a function of a transporter protein whose function can not be identified because of the inability to be transferred to a cell membrane in an exogenous gene expression system.

Other objectives will be known readily from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the amino acid sequences of mouse Asc-2 (SEQ ID NO: 1) and mouse Asc-1 (SEQ ID NO: 7), rat LAT1 (SEQ ID NO: 8), rat y+LAT1 (SEQ ID NO: 9), mouse xCT (SEQ ID NO: 10) and rat BAT1 (SEQ ID NO: 11) for comparison with each other. Twelve assumed membrane-spanning sites are designated with lines. A conserved cystine residue is designated with a *, and an assumed cAMP-dependent phosphorylation site with #.

FIG. 2 is a photograph substituted for the drawing showing the results of the in vitro translation of the Asc-2 (left) and the mouse 4F2hc (right). In the in vitro translation of the Asc-2 (left), a 55 kDa band was observed, revealing that no sugar was added in the presence of a canine pancreas microsome fraction (Microsomes) and the band size was not changed by a saccharide chain decomposition enzyme endoglycosidase (EndoH). On the contrary, the in vitro translation of the 4F2hc (right) exhibited a 65 kDa band, and a 80 kDa band was observed in the presence of canine pancreas microsome fraction. This band was reverted to a 65 kDa band by the saccharide chain digestion enzyme endoglycosidase H.

FIG. 3 is a photograph substituted for the drawing showing the results of the northern blotting analysis of the expression of the Asc-2 gene mRNA in various organs of a mouse. From the left of the figure, the brain, lung, heart, liver, spleen, skeletal muscle, kidney, small intestine, colon, testis and placenta are indicated in this order. The numbers on the left indicate the positions of the molecular weight markers.

FIG. 4 is a photograph substituted for the drawing showing the results of the northern blotting analysis of the expression of the Asc-2 gene mRNA (FIG. 4 a) and the expression of the Asc-1 gene mRNA (FIG. 4 b) in a mouse brain, a normal mouse spleen and a hemolytic anemia mouse spleen. The expression of the Asc-2 gene mRNA is higher in the hemolytic anemia mouse spleen (anemic spleen) than in the normal mouse spleen, while it was not observed in the brain (FIG. 4 a). On the contrary, the expression of the Asc-1 gene mRNA was observed only in the brain, and was not observed in either of the normal mouse spleen or the hemolytic anemia mouse spleen (FIG. 4 b).

FIG. 5 is a photograph substituted for the drawing showing the results of the western blotting analysis using an anti-Asc-2 antibody in a mouse erythrocyte membrane specimen (FIG. 5 a) and a mouse kidney membrane specimen (FIG. 5 b). The both were tested under a non-reducing condition (−) and a reducing condition (+).

FIG. 6 shows a schematic view of the structure of the fusion protein formed by connecting the Asc-2 to the rBAT or the 4F2hc. The cylindrical part represents a membrane-spanning region. The lower column of FIG. 6 indicates the amino acid sequences and the respective gene base sequences of the connection parts of the Asc-2-rBAT fusion protein (SEQ ID NOS 12-13) and the Asc-2-4F2hc fusion protein (SEQ ID NOS 14-15). Asc-2, SEQ ID NO: 1 (amino acid) SEQ ID NO: 2 (base): rBAT SEQ ID NO: 5 (amino acid) SEQ ID NO: 6 (base): 4F2hc SEQ ID NO:3 (amino acid) SEQ ID NO: 4 (base).

FIG. 7 shows the results of the experiment investigating the uptake of serine by an oocyte into which the Asc-2 gene cRNA, Asc-2 gene cRNA plus mouse 4F2hc gene cRNA, Asc-2-rBAT fusion protein gene cRNA and the Asc-2-4F2hc fusion protein gene cRNA were injected. The “water” indicated on the left end means a control into which water was introduced, and the ordinate of the graph indicates the serine uptake (pmol/oocyte cell/min).

FIG. 8 shows the results of the experiment investigating the uptake of serine by a COS-7 cell into which the Asc-2 gene, Asc-2 gene plus mouse rBAT gene and Asc-2 gene plus mouse 4F2hc gene were introduced. The left end shows the results of a control test (mock), and the ordinate of the graph indicates the serine uptake (pmol/cell/min).

FIG. 9 is a photograph substituted for the drawing showing the results of the fluoroimmunoassay of the expression of the fusion protein of the Asc-2 with the 4F2hc (Asc-2-4F2hc) in an oocyte cell membrane. A control oocyte into which water was injected (FIGS. 9 a and b), an oocyte into which the Asc-2 gene cRNA was injected and expressed (FIGS. 9 c and d) and an oocyte into which the cRNA of the gene of the fusion protein of the Asc-2 with the 4F2hc (Asc-2-4F2hc) was injected and expressed (FIGS. 9 e and f) were subjected to the test using an anti-4F2hc antibody (a, c and e) or an anti-Asc-2 antibody (b, d and f). The part exhibiting the fluorescence is designated by an arrow. The detected Asc-2 protein was not present on the cell membrane and was remaining within the cytoplasm (FIG. 9 d), while in the oocyte in which the fusion protein of the Asc-2 with the 4F2hc (Asc-2-4F2hc) was expressed both of the anti-4F2hc antibody (FIG. 9 e) and the anti-Asc-2 antibody (FIG. 9 f) detected the Asc-2-4F2hc fusion protein expressed on the cell membrane.

FIG. 10 shows the results of the experiment investigating the effects of salts added in the experiment of the serine uptake by the oocyte into which the cRNA of the gene of the fusion protein of the Asc-2 and the rBAT (Asc-2-rBAT) was injected. From the left, sodium chloride, choline chloride and sodium gluconate are indicated in this order. The ordinate of the graph indicates the serine uptake (pmol/oocyte/min).

FIG. 11 shows the results of the experiment investigating the effects of the substrate serine concentration in the experiment of the serine uptake by the oocyte into which the cRNA of the gene of the fusion protein of the Asc-2 and the rBAT (Asc-2-rBAT) was injected. The abscissa of the graph indicates the serine concentration (μM) while the ordinate indicates the serine uptake (pmol/oocyte/min).

FIG. 12 shows the results of the investigation of the release of ¹⁴C-serine from an oocyte into which the cRNA of the gene of the fusion protein of the Asc-2 and the rBAT (Asc-2-rBAT) was injected in the presence (+) or absence (−) of the extracellular non-labeled serine (100 μM). The ordinate indicates a released radioactivity as % based on the radioactivity injected into the oocyte. The solid column indicates a control oocyte into which water was injected instead of the cRNA, and the hatched column indicates the oocyte into which the Asc-2-rBAT gene cRNA was injected. The designation “*” or “**” indicates a significant difference.

FIG. 13 shows the results of the investigation of the effects of the addition of various L-amino acids or their analogues to the system in the experiment of the serine uptake by the oocyte into which the cRNA of the gene of the fusion protein of the Asc-2 and the rBAT (Asc-2-rBAT) was injected. In FIG. 13, the indication (−) means no addition, AIB means α-aminoisobutyric acid, MeAIB means α-methylaminoisobutyric acid, GABA means γ-aminobutyric acid and BCH means 2-amino-2-norbornanecarboxylic acid. The ordinate indicate a ratio of the uptake level with the value of (−) being regarded as 100%.

FIG. 14 shows the results of the radiolabeled amino acid uptake by the oocyte into which the cRNA of the gene of the fusion protein of the Asc-2 and the rBAT (Asc-2-rBAT) was injected. AIB means α-aminoisobutyric acid and MeAIB means α-methylaminoisobutyric acid. The ordinate indicates the serine uptake (pmol/oocyte/min).

FIG. 15 is a photograph substituted for the drawing showing the results of the immunohistological analysis of the Asc-2 using an anti-Asc-2 antibody in a mouse kidney. The photo a shows a slightly magnified image. An intense staining was observed in a collecting tubule in the area from the outer layer to the inner layer of a medulla. The photo b shows the results of the absorption test using an antigen peptide. The staining observed in the photo a was disappeared, and the specificity of the staining was observed. The photo c is a highly magnified image of the cortical collecting tubule. The staining of the epithelium of the collecting tubule was noted. The photo d is a highly magnified image of the collecting tubule of the outer layer of the medulla. Luminal and basal membranes of the epithelium of the collecting tubule were stained. The photos e and f are highly magnified images of the collecting tubule of the inner layer of the medulla. Luminal and basal membranes of the epithelium of the collecting tubule were stained. In FIG. 15, the designation C means a renal cortex, OM means a renal medulla outer layer, and IM means a renal medulla inner layer.

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors searched an EST (expressed sequence tag) database using the base sequence of the translation region of the cDNA of a LAT1 and identified a base sequence analogous to the LAT1. The base sequence of the corresponding cDNA clone was determined and was proven to encode a novel protein. In addition, a fusion protein of a translation product of this gene with a 4F2hc or rBAT was prepared, and expressed on a cell membrane of an oocyte of a Xenopus. As a result, the function of the translation product of this gene was proven to correspond to a neutral amino acid transport system asc and, unlike to an already known Asc-1, also correspond to a traditional transport system asc which has previously been reported with regard to an erythrocyte membrane, whereby the invention was established.

Thus, the invention is a protein selected from the group consisting of the following (A) and (B):

-   (A) a protein consisting of the amino acid sequence represented by     SEQ ID NO.1; and, -   (B) a protein having an ability to transport a small-sized neutral     amino acid and its analogue in a sodium-independent manner which     consists of an amino acid sequence formed as a result of the     deletion, substitution or addition of one or more amino acids in the     amino acid sequence represented by SEQ ID NO.1.

Also the invention is a gene consisting of a DNA selected from the group consisting of the following (a) and (b):

-   (a) a DNA consisting of the base sequence represented by SEQ ID     NO.2; and, -   (b) a DNA encoding a protein having an ability to transport a     small-sized neutral amino acid and its analogue in a     sodium-independent manner which hybridizes under a stringent     condition with a DNA consisting of the base sequence represented by     SEQ ID NO.2.

Also the invention is a method for analyzing a function of a transporter protein by allowing the transporter protein, whose function can not be identified because of the inability to be transferred to a cell membrane in an exogenous gene expression system, to be transferred to the cell membrane by converting said protein into a fusion protein with a protein having the amino acid sequence represented by SEQ ID NO.3 (4F2hc) or a protein having the amino acid sequence represented by SEQ ID NO.5 (rBAT).

A novel protein having an ability to transport a small-sized neutral amino acid and its analogue in a sodium-independent manner, i.e., an amino acid transporter Asc-2 (asc-type amino acid transporter 2) can be converted into a fusion protein with the 4F2hc or rBAT whereby being expressed on a cell membrane and imparted with an ability to transport a small-sized neutral amino acid such as glycine, L-alanine, L-serine, L-cysteine and L-threonine with a high affinity. In addition, L-methionine, L-leucine, L-isoleucine, L-valine, L-phenylalanine, L-tyrosine, L-histidine, D-serine and D-alanine are also transported at a low affinity.

An inventive sodium-independent transporter Asc-2 which transports a small-sized neutral amino acid is expressed in vivo mainly in a kidney, placenta and skeletal muscle. A low expression is noted also in a spleen and lung.

SEQ ID NO.1 in the sequence listing described below represents the amino acid sequence of a mouse-derived sodium-independent transporter (mouse Asc-2) which transports a small-sized neutral amino acid, and SEQ ID NO.2 represents the full-length cDNA base sequence (about 1.8 kbp) of the respective gene together with the amino acid sequence (465 amino acids) of a protein encoded by the respective translation region.

As a result of the homology search of all sequences included in known DNA database (GenBank and EMBL) and protein database (NBRF and SWISS-PROT) for the amino acid sequence represented by SEQ ID NO.1 and the base sequence represented by SEQ ID NO.2, there are no identical sequences, suggesting that the sequences are novel.

A protein of the invention may, for example, be one having the amino acid sequence represented by SEQ ID NO.1 as well as a protein having an amino acid sequence formed as a result of the deletion, substitution or addition of one or more amino acids in the amino acid sequence represented by SEQ ID NO.1. The number of the amino acids undergoing the deletion, substitution or addition may be any as long as causing no loss of the neutral amino acid-transporting ability, and usually 1 to about 93, preferably 1 to about 47. Such a protein has an amino acid sequence homology usually of 1 to 80%, preferably 1 to 90% with the amino acid sequence represented by SEQ ID NO.1.

An inventive gene may, for example, be one having the base sequence represented by SEQ ID NO.2 as well as the one comprising a DNA which can hybridize under a stringent condition with a DNA consisting of the base sequence represented by SEQ ID NO.2. The DNA which can hybridize may be anyone as long as the protein encoded by the DNA has an ability to transport a neutral amino acid. Such a DNA has a base sequence homology usually of 70% or more, preferably 80% or more with the base sequence represented by SEQ ID NO.1. Such a DNA includes a naturally occurring variant gene, artificially modified variant gene, heterogeneous organism-derived homologous gene and the like.

In the invention, a hybridization under a stringent condition can be effected usually by conducting a hybridization for about 12 hours at a temperature of 37 to 42° C. in a hybridization solution whose salt concentration is 5×SSC or equivalent thereto followed by a preliminary washing if necessary with the solution whose salt concentration is 5×SSC or equivalent thereto, and by washing with the solution whose salt concentration is 1×SSC or equivalent thereto.

An inventive sodium-independent transporter gene transporting small-sized neutral amino acid can be isolated by conducting a screening using as a gene source a tissue or a cell of suitable mammalian animals. Mammalian animals may for example be non-human animals such as dog, cattle, horse, goat, sheep, monkey, pig, rabbit, rat and mouse, as well as human.

The gene screening and isolation can be conducted preferably by a homology cloning method and the like.

For example, a mouse or human kidney is employed as a gene source, from which an mRNA (poly(A)⁺RNA) is prepared and used to construct a cDNA library, which is then screened using a probe corresponding to a LAT1-analogous sequence obtained by searching the EST (expressed sequence tag) data base (for example, GenBank™/EBI/DDBJ accession No. AI875555), whereby obtaining a clone containing an Asc-2 gene cDNA.

The base sequence of the resultant cDNA is determined by a standard method, and its translation region is analyzed to determine the protein encoded thereby, i.e., the amino acid sequence of the Asc-2.

Whether the resultant cDNA is a sodium-independent transporter gene transporting small-sized neutral amino acid or not, i.e., whether the gene product encoded by the cDNA is a sodium-independent transporter gene transporting small-sized neutral amino acid or not, can be verified for example as follows. Thus, based on the cDNA of the resultant Asc-2 gene, a cDNA encoding the fusion protein of the Asc-2 with a 4F2hc or rBAT is prepared and used to prepare an RNA complementary thereto (cRNA) (capped), which is introduced into and expressed in an oocyte. Then an ability to transport (take in) a neutral amino acid into the cell is verified by measuring the uptake of a substrate into the cell in accordance with a standard uptake test (Kanai and Hediger, Nature, Vol. 360, page 467-471, 1992) using a suitable neutral amino acid as a substrate.

An RNA complementary to the resultant Asc-2 gene cDNA is prepared and subjected to an in vitro translation method [Hediger et al., Biochim. Biophys. Acta, Vol. 1064, page 360, 1991] to synthesize an asc-1 protein, which is then examined for the size of the protein and the presence or absence of glycosylation by means of an electrophoresis.

Since the 4F2hc gene cDNA has already been reported [Fukasawa et al., Biol. Chem. 275: 9690-9698, 2000], this sequence data may be employed to obtain the 4F2hc gene easily for example by a PCR method.

SEQ ID NO.3 in the sequence listing described below represents the amino acid sequence (526 amino acids) of a mouse-derived 4F2hc, and SEQ ID NO.4 represents the full-length cDNA base sequence (about 1.8 kbp) of the respective gene together with the amino acid sequence of a protein encoded by the respective translation region.

Since the rBAT gene cDNA has also been reported [Segawa, H. et al., J. Biochem. J. 328: 657-664, 2000], this sequence data may be employed to obtain the rBAT gene easily for example by a PCR method.

SEQ ID NO.5 in the sequence listing described below represents the amino acid sequence (685 amino acids) of a mouse-derived rBAT, and SEQ ID NO.6 represents the full-length cDNA base sequence (about 2.3 kbp) of the respective gene together with the amino acid sequence of a protein encoded by the respective translation region.

A cDNA encoding the fusion protein of the Asc-2 with the 4F2hc or rBAT can readily be prepared for example by a PCR method based on the cDNA of the Asc-2 gene, cDNA of the 4F2hc gene or cDNA of the rBAT gene.

An expressing cell may be subjected to a similar uptake experiment to examine for a property of the Asc-2, such as a transport of the amino acids as a combination of the exchange transport type and a facilitated diffusion type as well as for the substrate selectivity and the pH dependency of the Asc-2.

Using the resultant Asc-2 gene cDNA, an appropriate cDNA library or genomic DNA library produced from a different gene source may be screened to isolate a homologous gene or chromosomal gene derived from a different tissue or different organism.

It is also possible to isolate the gene from a cDNA library or genomic DNA library by an ordinary PCR (polymerase chain reaction) method using a synthetic primer designed based on the disclosed data of the base sequence of an inventive gene (base sequence represented by SEQ ID NO.2 or its part).

A DNA library such as a cDNA library or genomic DNA library can be produced in accordance with the method described for example in Molecular Cloning (Sambrook, J., Fritsh, E. F. and Manitis, T., Cold Spring Harbor Press, 1989). Alternatively, a commercially available library, if any, may be employed.

An inventive sodium-independent transporter transporting small-sized neutral amino acid and its gene (Asc-2) can be produced for example by a gene recombination technology using a cDNA encoding it. For example, a DNA (such as cDNA) encoding the Asc-2 is integrated into a suitable expression vector, and the resultant recombinant DNA can be introduced into a suitable host cell. The expression system for producing a polypeptide (host-vector system) may for example be expression systems of bacteria, yeasts, insect cells and mammalian cells. Among these, the insect cells and the mammalian cells are preferred for the purpose of obtaining a functional protein.

A fusion protein of an inventive sodium-independent transporter transporting small-sized neutral amino acid and the 4F2hc or rBAT and their genes (Asc-2-4F2hc or Asc-2-rBAT) can be produced, for example, by a gene recombination technology using a cDNA encoding it. For example, a DNA (such as cDNA) encoding the Asc-2-4F2hc or Asc-2-rBAT is integrated into a suitable expression vector, and the resultant recombinant DNA can be introduced into a suitable host cell. The expression system for producing a polypeptide (host-vector system) may for example be expression systems of bacteria, yeasts, insect cells and mammalian cells. Among these, the insect cells and the mammalian cells are preferred for the purpose of obtaining a functional protein.

For example, to express a polypeptide in a mammalian cell, a DNA encoding an inventive sodium-independent transporter Asc-2 transporting small-sized neutral amino acid or a fusion protein of the Asc-2 with the 4F2hc or rBAT is inserted into the downstream of a suitable promoter (such as cytomegalovirus promoter, SV40 promoter, LTR promoter, elongation 1a promoter and the like) in a suitable expression vector (such as adenovirus vector, retrovirus vector, papilloma virus vector, vaccinia virus vector, SV40 vector, various plasmids and the like) to construct an expression vector. Then, with the resultant expression vector, a suitable animal cell is transformed to obtain a transformant which is then cultured in a suitable medium to allow the intended polypeptide to be produced. A mammalian cell serving as a host cell may for example be a cell line such as simian COS-7 cell, Chinese hamster CHO cell or human HeLa cell.

Accordingly, the invention provides a vector comprising an inventive gene described above or a gene encoding a protein in said gene, preferably an expression vector, as well as a host cell (transformant) which has been transformed with said vector.

A DNA encoding a sodium-independent transporter Asc-2 transporting small-sized neutral amino acid may for example be a cDNA having the base sequence represented by SEQ ID NO.1 as well as any DNA encoding the polypeptide designed based on the amino acid sequence without any limitation to the cDNA described above. In such a case, a codon encoding a single amino acid is known to present in 1-6 types respectively, any of which may be employed, and a sequence habing further higher expression efficiency can be designed while taking the frequency of the use of the codon by a host employed for the expression into consideration. A DNA having a designed base sequence can be obtained by a chemical synthesis of the DNA, fragmentation and binding of the cDNA described above, partial alteration of the base sequence and the like. The partial alteration of the base sequence or the introduction of a variation can be artificially accomplished by means of a site specific mutagenesis method [Mark, D. F. et al., Proceedings of National Academy of Sciences, Vol. 81, page 5662, (1984)] and the like utilizing the primers consisting of the synthetic oligonucleotides encoding the intended alteration.

A DNA encoding a fusion protein of a sodium-independent transporter Asc-2 transporting small-sized neutral amino acid with the 4F2hc or rBAT (Asc-2-4F2hc or Asc-2-rBAT) can be prepared for example by using a cDNA having the base sequence represented by SEQ ID NO.2 and the base sequence represented by SEQ ID NO.4 or the base sequence represented by SEQ ID NO.6, and it is also possible to use any DNA encoding the polypeptide designed based on the amino acid sequence without any limitation to the cDNA described above. In such a case, a codon encoding a single amino acid is known to present in 1-6 types respectively, any of which may be employed, and a sequence having further higher expression efficiency can be designed while taking the frequency of the use of the codon by a host employed for the expression into consideration. A DNA having a designed base sequence can be obtained by a chemical synthesis of the DNA, fragmentation and binding of the cDNA described above, partial alteration of the base sequence and the like. The partial alteration of the base sequence or the introduction of a variation can be artificially accomplished by means of a site specific mutagenesis method [Mark, D. F. et al., Proceedings of National Academy of Sciences, Vol. 81, page 5662, (1984)] and the like utilizing the primers consisting of the synthetic oligonucleotides encoding the intended alteration.

Accordingly, the invention provides a nucleotide comprising a partial sequence of consecutive 14 bases or more, preferably 20 bases or more, in the base sequence represented by SEQ ID NO.2 or a sequence complementary thereto.

An inventive nucleotide can be used as a probe for detecting a gene encoding a protein having an ability to transport a small-sized neutral amino acid and its analogue in a sodium-independent manner, and also as a primer in obtaining a gene encoding said protein or a gene encoding a protein highly homologous to said protein, and further can be used to modulate the expression of a gene encoding a protein having an ability to transport a small-sized neutral amino acid and its analogue in a sodium-independent manner for example by utilizing its antisense strand.

Using an inventive sodium-independent transporter transporting small-sized neutral amino acid or its immunologically equivalent polypeptide, its antibody can be obtained. The antibody can be utilized in detecting or purifying a sodium-independent transporter transporting small-sized neutral amino acid. The antibody can be produced by using as an antigen an inventive sodium-independent transporter transporting small-sized neutral amino acid, its fragment or synthetic peptide having its partial sequence. A polyclonal antibody can be produced by an ordinary method in which an antigen is inoculated into a host animal (such as rat and rabbit) and then an immune serum is recovered, while a monoclonal antibody can be produced by an ordinary hybridoma method.

Accordingly, the invention provides an antibody directed to a protein of the invention described above, preferably, a specific antibody directed to said protein.

An inventive protein has an ability to transport a small-sized neutral amino acid and its analogue in a sodium-independent manner, and such an ability is influenced potently by the presence of various substances. By screening for a substance inhibiting or promoting this ability, a control of the ability of the inventive protein to transport the substance becomes possible.

Accordingly, the invention provides a method for detecting an effect of a test substance serving as a substrate on an ability to transport a small-sized neutral amino acid and its analogue in a sodium-independent manner possessed by an inventive protein described above by means of employing said protein.

An amino acid transported by an inventive protein is a substance essential for the proliferation, growth and survival of a cell, and by controlling the uptake of such a substance into the cell, the proliferation and the growth of the cell can be controlled. Accordingly, the invention provides a method for controlling a cell proliferation by modulating an ability to transport a small-sized neutral amino acid and its analogue possessed by an inventive protein described above by means of employing said protein, its specific antibody, or its function-promoting substance or function-suppressing substance.

The gene of a fusion protein of an inventive sodium-independent transporter Asc-2 transporting small-sized neutral amino acid with the 4F2hc or rBAT and its expression cell can be used in an in vitro permeability test of a substance in the cell membrane where the Asc-2 is present or in a site where the Asc-2 is assumed to be present. The gene of a fusion protein of an inventive sodium-independent transporter Asc-2 transporting small-sized neutral amino acid with the 4F2hc or rBAT and its expression cell can be used also in developing a compound which permeates efficiently through the cell membrane where the Asc-2 is present or through a site where the Asc-2 is assumed to be present. Furthermore, the gene of a fusion protein of an inventive sodium-independent transporter Asc-2 transporting small-sized neutral amino acid with the 4F2hc or rBAT and its expression cell can be used in an in vitro inter-pharmaceutical interaction test in the cell membrane where the Asc-2 is present or in a site where the Asc-2 is assumed to be present.

Accordingly, the invention provides a method for altering the in vivo pharmacokinetics of a pharmaceutical, toxic substance or exogenous foreign body transported by an inventive protein described above by modulating an ability to transport a neutral amino acid and its analogue possessed by said protein by means of employing said protein, its specific antibody, or its function-promoting substance or function-suppressing substance.

Since an inventive protein has an ability to transport a small-sized neutral amino acid and its analogue in a sodium-independent manner and this ability can be suppressed not only by the number of the proteins present in a cell but also by the presence of various substances (in the presence of such as function-suppressing substance) and can also be promoted (in the presence of such as function-promoting substance) as described above, the invention provides an agent for controlling an ability to transport a small-sized neutral amino acid and its analogue possessed by an inventive protein described above which comprises said protein, its specific antibody or its function-promoting substance or function-suppressing substance.

An inventive transporting ability-controlling agent can be used as a cell proliferation-controlling agent because of its ability to control the proliferation and the growth of a cell and also as an agent for controlling the in vivo pharmacokinetics of a pharmaceutical, toxic substance or xenobiotics because of its ability to modulate and control the in vivo pharmacokinetics of the pharmaceutical, toxic substance or xenobiotics.

By suppressing an inventive sodium-independent transporter Asc-2 transporting small-sized neutral amino acid, the permeation of a certain compound through the cell membrane where the Asc-2 is expressed or through a site where the Asc-2 is assumed to be present can be suppressed. Furthermore, the gene of a fusion protein of an inventive sodium-independent transporter Asc-2 transporting small-sized neutral amino acid with the 4F2hc or rBAT and its expression cell can be used in developing a pharmaceutical which suppresses the permeation of a compound transported by the Asc-2 through the cell membrane and also the permeation through the site where the Asc-2 is assumed to be present (Asc-2-specific inhibitor and the like).

Accordingly, the invention also provides a method for analyzing a function of a transporter protein comprising a step for allowing a protein, whose function can not be identified because of the inability to be transferred to a cell membrane in an exogenous gene expression system, to be transferred to the cell membrane by converting said protein into a fusion protein with a protein which promotes the transfer to the cell membrane. An inventive protein which promotes the transfer to the cell membrane is preferably a protein having the amino acid sequence represented by SEQ ID NO.3 or 5 or a protein consisting of an amino acid sequence formed as a result of the deletion, substitution or addition of one or more amino acids in said protein. A protein whose function can not be identified because of the inability to be transferred to a cell membrane in an exogenous gene expression system is preferably but not limited to a transporter protein.

The inventors also found that a protein having an amino acid sequence represented by SEQ ID NO.3 or NO.5 has an ability to promote the transfer of a protein into a cell membrane, and accordingly the invention also provides an agent for promoting the transfer of a protein into a cell membrane comprising a protein having an amino acid sequence represented by SEQ ID NO.3 or NO.5 or a protein consisting of an amino acid sequence formed as a result of the deletion, substitution or addition of one or more amino acids in said protein. An inventive protein to be transferred to the cell membrane is preferably a transporter protein whose function can not be identified because of the inability to be transferred to a cell membrane in an exogenous gene expression system.

EXAMPLES

The invention is further described in detail by the following EXAMPLES which are not intended to restrict the invention.

Unless otherwise specified, each procedure of the following EXAMPLES was conducted in accordance with the methods described in Molecular Cloning (Sambrook, J., Fritsh, E. F. and Manitis, T., Cold Spring Harbor Press, 1989) or a manufacture's instruction when using a commercial reagent or kit.

Example 1

(1) Identification of Mouse cDNA of Sodium-independent Transporter Transporting Small-sized Neutral Amino Acid

A cDNA clone, corresponding to a mouse-derived base sequence GenBank™/EBI/DDBJ accession No. AI875555 analogous to the rat LAT1 obtained by searching the EST (expressed sequence tag) database employing the base sequence of the translation region of the rat LAT1 [Kanai et al., J. Biol. Chem. 273: 23629-23632, 1998], was purchased from IMAGE (Integrated and Molecular Analysis of Genomes and their Expression) (IMAGE clone I.D.: 1972372), and subjected to a dye terminator cycle sequencing method (Applied Biosystems) using synthetic primers for a base sequencing to determine the full-length base sequence of the cDNA. The base sequence of the cDNA was analyzed by an ordinary method to determine the translation region of the cDNA and the amino acid sequence of a protein encoded thereby.

This amino acid sequence is represented by SEQ ID NO.1 in the following sequence listing and the base sequence is represented by SEQ ID NO.2.

The Asc-2 comprised the 34% homology of amino acid sequence with a rat transporter LAT1 corresponding to a neutral amino acid transport system L and 33% with the LAT2. The Asc-2 also comprised the 34% homology with a rat transporter y⁺LAT1 corresponding to a neutral and basic amino acid transport system y⁺L and 32% with a human transporter y⁺LAT2. Further, the Asc-2 comprised the 34% homology of amino acid sequence with a mouse transporter xCT corresponding to a cystine and acidic amino acid transport system x⁻C and 35% with a rat transporter BAT1 corresponding to a cystine and neutral and basic amino acid transport system b^(0,+). Furthermore, the Asc-2 comprised a 28 to 29% homology with mouse and human transporter CAT 1 to 4 corresponding to a basic amino acid transport system y⁺.

The amino acid sequences of mouse Asc-2 and mouse Asc-1, ratLAT1, rat y⁺LAT1, human y⁺LAT2, mouse xCT and rat BAT1 were compared in FIG. 1.

Based on a SOSUI algorithm [Hirokawa, T. et al., Bioinformatics, Vol. 14, page 378 (1998)], the amino acid sequence of the Asc-2 was analyzed and 12 membrane-spanning domains were assumed as shown in FIG. 1. The 3rd hydrophilic loop contained a cysteine residue conserved among the LAT1, LAT2, Asc-2, y⁺LAT1, y⁺LAT2, xCT and BAT1. This cysteine residue was assumed to bind the Asc-2 to an unknown cofactor via its disulfide bond. The 6th hydrophilic loop contained a site which was assumed to a cAMP-dependent phosphorylation site.

(2) Asc-2 Protein Analysis by In Vitro Translation

By means of an in vitro translation method [Hediger et al., Biochim. Biophys. Acta, Vol. 1064, page 360, 1991], Asc-2 and 4F2hc proteins were synthesized from the Asc-2 cRNA and a mouse 4F2hc cRNA, and subjected to an electrophoresis.

The results are shown as a photograph substituted for the drawing in FIG. 2. As evident from this, a 55 kDa band was observed, which proved that no sugar was added in the presence of a canine pancreas microsome fraction containing a series of sugar addition enzymes, indicating a protein having no sugar addition site (see FIG. 2). On the contrary, the in vitro translation of the 4F2hc cRNA conducted as a control of the sugar addition reaction exhibited a 65 kDa band, and a 80 kDa band was observed in the presence of canine pancreas microsome fraction. This band was reverted to a 65 kDa band by the saccharide chain cleavage endoglycosidase H.

(3) Expression of Asc-2 Gene in Various Tissues of Mouse (Analysis by Northern Blotting)

A cDNA fragment corresponding to the 135th to 735th base pairs of the Asc-2 gene was amplified by a PCR, labeled with ³²P-dCTP, and used as a probe to conduct the northern blotting for the RNA extracted from various tissues of a mouse as described below. 3 μg of a poly(A)⁺RNA was subjected to an electrophoresis on a 1% agarose/formaldehyde gel and transferred onto a nitrocellulose filter. This filter was subjected to a hybridization overnight in a hybridization solution containing a cDNA fragment of the Asc-2 labeled with ³²P-dCTP at 42° C. The filter was washed with 0.1×SSC containing 0.1% SDS at 65° C.

The results of the northern blotting are shown as a photograph substituted for the drawing in FIG. 3. As a result, the kidney exhibited a band near 1.7 kb. The kidney and the placenta exhibited a band near 4.0 kb. A less intense band near 7.5 kb was exhibited by the lung and the spleen. The skeletal muscle exhibited short bands near 0.5 kb and 1.0 kb.

Also for the purpose of examining whether the Asc-2 is expressed in an erythrocyte or not, a method by Hara et al (Hara, H. and Ogawa. M.: Am. J. Hematol. 1: 453-458, 1976) was employed to develop a hemolytic anemia in a male ICR mouse by administering 1-acetyl-2-phenylhydrazine (60 mg/kg body weight) on the 0th, 1st and 3rd days whereby inducing a splenic hematopoiesis, and then the spleen was taken out on the 6th day. 3 μg of the poly(A)⁺RNA extracted from this spleen was subjected together with the poly(A)⁺RNA (3 μg) of the spleen derived from a non-treated male ICR mouse to an electrophoresis on a 1% agarose/formaldehyde gel and transferred onto a nitrocellulose filter. This filter was subjected to a hybridization overnight in a hybridization solution containing a cDNA fragment of the Asc-2 labeled with ³²P-dCTP at 42° C. The filter was washed with 0.1×SSC containing 0.1% SDS at 65° C. At the same time, a northern hybridization using as a probe a cDNA fragment of a mouse Asc-1 labeled with ³²P-dCTP was conducted.

The results of this northern blotting are shown as a photograph substituted for the drawing in FIG. 4. As a result, the both of the non-treated mouse spleen and the 1-acetyl-2-phenylhydrazine-treated mouse spleen exhibited a band near 7.5 kb. The expression in the 1-acetyl-2-phenylhydrazine-treated mouse was higher by 3.2±0.6 (mean±standard error, n=3) than that in the non-treated mouse. Accordingly, it was suggested that the Asc-2 was expressed in the erythrocyte. On the contrary, the Asc-1 was expressed only in the brain, and no expression in the spleen was noted either in the non-treated mouse or in the 1-acetyl-2-phenylhydrazine-treated mouse.

(4) Expression of Asc-2 Protein in Mouse Erythrocyte and Mouse Kidney

A specific antibody directed to a synthetic oligopeptide corresponding to the 455-465 of a mouse Asc-2 [SPSEDPEEQKNC] (cysteine residue at the C-terminal was introduced for the conjugation with KLH (keyhole limpet hemocyanine)) (SEQ ID NO: 16) was prepared in accordance with a method by Altman et al (Altman et al., Proc. Natl. Acad. Sci. USA, Vol. 81, page 2176-2180, 1984).

Membrane fractions of the erythrocyte and the kidney of a mouse were prepared in accordance with a method by Thorens et al [Thorens et al., Cell Vol. 55, page 281 to 290, 1988]. A protein sample was treated for 5 minutes at 100° C. in the presence (reducing condition) or absence (non-reducing condition) of 5% 2-mercaptoethanol, subjected to an electrophoresis on an SDS-polyacrylamide gel, blotted onto a Hybond-P PVDV transfer membrane, and treated with an anti-Asc-2 affinity purified antibody (1:5000).

The results are shown as a photograph substituted for the drawing in FIG. 5. The mouse erythrocyte exhibited, in response to the anti-Asc-2 antibody, the bands near 80 kDa, 200 kDa and 250 kDa under the non-reducing condition as shown in FIG. 5. The mouse kidney exhibited a band near 90 kDa. Under the reducing condition, both of the erythrocyte and the kidney exhibited a band near 60 kDa. Based on these results, the Asc-2 was suggested to be bound to some protein via a disulfide bond. In addition, it is also suggested that there is a difference between the erythrocyte and the kidney in the protein bound to the Asc-2.

Example 2 Preparation of Fusion Protein of Sodium-independent Transporter Asc-2 Transporting Small-sized Neutral Amino Acid with 4F2hc or rBAT and Analysis of its Function

(1) Preparation of Fusion Protein of Sodium-independent Transporter Asc-2 Transporting Small-sized Neutral Amino Acid with 4F2hc or rBAT

A fusion protein of the Asc-2 with the rBAT (Asc-2-rBAT) was prepared by using synthetic oligo DNA primers 5′-GCGCGAATTCAAGCTTGAACACCCTGTTTGACAGGG-3′ (SEQ ID NO: 17) (a sequence corresponding to the 17th to 36th base pairs of a cDNA of the Asc-2 combined with a sequence corresponding to the HindIII and EcoRI cleavage sites and GCGC at the 5′-terminal) and 5′-GCGCGAATTCACTAGTATTTTTCTGTTCTTCTGGAT-3′ (SEQ ID NO: 18) (a sequence corresponding to the 1491st to 1510th base pairs of a cDNA of the Asc-2 combined with a sequence corresponding to the SpeI and EcoRI cleavage sites and GCGC at the 5′ terminal) together with a cDNA of the Asc-2 as a template to conduct a PCR. The resultant PCR product was cleaved with HindIII and EcoRI and ligated to the HindIII and EcoRI sites of a mammalian cell expression vector pcDNA3.1(+) (Invitrogen). On the other hand, synthetic oligo DNA primer 5′-GCGCATCGATAGCCAGGACACCGAAGTGGA-3′ (SEQ ID NO: 19) (a sequence corresponding to the 20 base pairs immediately after the translation initiation codon ATG of the mouse rBAT represented by SEQ ID NO.6 combined with a sequence corresponding to the EcoRI cleavage site and GCGC at the 5′ terminal) and 5′-GCGCGCGGCCGCCATATTTAAATGCTTTAGTA-3′ (SEQ ID NO: 20) (a sequence corresponding to the 2240th to 2259th base pairs of the mouse rBAT represented by SEQ ID NO.6 combined with a sequence corresponding to the NotI cleavage site and GCGC at the 5′ terminal) were employed together with a cDNA of the rBAT as a template to conduct a PCR. The resultant PCR product was cleaved with EcoRI and NotI and ligated to the EcoRI and NotI sites of a mammalian cell expression vector pcDNA3.1 (+) into which the PCR product of the Asc-2 had been integrated as described above to obtain a cDNA encoding the fusion protein of the Asc-2 with the 4F2hc (see FIG. 6).

(2) The results of the ¹⁴C-serine uptake test in oocytes are shown in FIG. 7. The levels of the ¹⁴C-serine uptake in the oocyte where only the Asc-2 was expressed and the oocyte where the Asc-2 and the 4F2hc were co-expressed were similar to that in the control oocyte into which water was injected, while a higher serine uptake was noted in the oocyte where the Asc-2-rBAT or the Asc-2-4F2hc was expressed.

It was examined that the rBAT or the 4F2hc does not serve as a direct cofactor for the Asc-2 using a COS-7 cell. A plasmid DNA (each 1 μg) containing the cDNA of the Asc-2, cDNA of the rBAT or cDNA of the 4F2hc was introduced into the COS-7 cell using a Lipofectamine 2000 reagent (life Technologies) in accordance with a method by Mizoguchi et al (Kidney Int. 59: 1821-1833, 2001]. After the introduction, the cell was incubated in a 24-well plate for 2 days and then the uptake of ¹⁴C-serine (10 μM) was measured. In accordance with the method of Mizoguchi et al. [Mizoguchi et al.: Kidney Int. 59: 1821-1833, 2001], the measurement of the uptake was initiated by removing the culture medium and adding a Dulbecco's PBS (Gibco) containing ¹⁴C-serine, which was subsequently removed and the measurement was terminated by washing the plate with an ice-cooled Dulbecco's PBS. After the washing it was dissolved in 0.1N NaOH, and the radioactivity was measured by a liquid scintillation counting.

The results are shown in FIG. 8. The levels of the serine uptake in the oocyte where only the Asc-2 was expressed and the oocyte where the Asc-2 and the rBAT were co-expressed and also in the oocyte where the Asc-2 and the 4F2hc were co-expressed were similar to that in the control oocyte into which water was injected. It proved that the rBAT or the 4F2hc does not serve as a direct cofactor for the Asc-2.

(3) Identification of Expression of Fusion Protein of Sodium-independent Transporter Asc-2 Transporting Small-sized Neutral Amino Acid with 4F2hc (Asc-2-4F2hc) in Oocyte Cell Membrane by Fluoroimmunoassay

Whether the fact that no function was observed when allowing Asc-2 to express in oocyte but the fusion protein of the Asc-2 with the 4F2hc (Asc-2-4F2hc) exhibited a functional activity is attributable to the inability of the Asc-2 to be transported to the cell membrane and the ability of the Asc-2-4F2hc to be transported to the cell membrane or not was verified by a fluoroimmunoassay.

25 ng of the Asc-2 gene cRNA or 25 ng of the cRNA of the gene of the fusion protein of the Asc-2 with the 4F2hc (Asc-2-4F2hc) was injected into an oocyte and allowed to be expressed, and after incubating for 3 days the oocyte was fixed in a 4% paraformaldehyde-phsophate buffer solution, and subjected to an ordinary method to obtain a paraffin section (3 μm). After removing the paraffin, the section was blocked with 5% goat serum in a 0.05M tris-buffered physiological saline containing 0.1% Tween 20, and then treated with an affinity-purified anti-Asc-2 antibody or an affinity-purified anti-4F2hc antibody [Fukasawa et al., J. Biol. Chem. 275: 9690-9698, 2000]. Thereafter, the section was treated with a Cy3-labeled goat anti-rabbit IgG (Jacson ImmunoResearch Laboratories), washed with a 0.05M tris-buffered physiological saline containing 0.1% Tween 20, and then observed with an Olympus Fluoview (FV 500) confocal laser microscope (Olympus). The excitation was effected with a Green Hanere laser at 543 nm, and the fluorescence from the Cy3 was detected using a BA560IF filter.

The results are shown as a photograph substituted for the drawing in FIG. 9. The Asc-2 protein detected by the anti-Asc-2 antibody in the oocyte where the Asc-2 was expressed was not present on the cell membrane and was remaining within the cytoplasm (FIG. 9 d), while in the oocyte where the fusion protein of the Asc-2 with the 4F2hc (Asc-2-4F2hc) was expressed the both of the anti-4F2hc antibody (FIG. 9 e) and the anti-Asc-2 antibody (FIG. 9 f) allowed the Asc-2-4F2hc fusion protein expressed on the cell membrane to be detected. The control oocyte into which water was injected exhibited no specific color development in response to the anti-4F2hc antibody (FIG. 9 a) or the anti-Asc-2 antibody (FIG. 9 b). Accordingly, it was proven that the fact that no function was observed when allowing the oocyte to express the Asc-2 while the fusion protein of the Asc-2 with the 4F2hc (Asc-2-4F2hc) exhibited a functional activity is attributable to the inability of the Asc-2 to be transported to the cell membrane and the ability of the fusion protein with the 4F2hc (Asc-2-4F2hc) to be transported to the cell membrane.

(4) Salt Dependency of Asc-2 Transport Activity

In a serine uptake test in an oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 with the 4F2hc or the rBAT (Asc-2-4F2hc or Asc-2-rBAT) was injected, the effect of a salt added to a culture medium was investigated.

The serine uptake test was conducted using the oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 with the 4F2hc or the rBAT (Asc-2-4F2hc or Asc-2-rBAT) was injected in accordance with the method in Section (2) in Example 2 described above. Nevertheless, the test was conducted using as an uptake solution for investigating the effect of sodium a standard uptake solution (100 mM sodium chloride instead of 100 mM choline chloride) instead of the sodium-free uptake solution (Na+-free uptake solution). The test was conducted also using as an uptake solution for investigating the effect of chloride ion a gluconic acid uptake solution (100 mM sodium gluconate instead of 100 mM sodium chloride) instead of a standard uptake solution.

The results are shown in FIG. 10. Even when exchanging the extracellular choline with sodium, or when exchanging the extracellular chloride ion with the gluconate ion, the serine uptake was not influenced at all. Accordingly, it was suggested that the Asc-2 is a transporter which acts independently of the sodium ion or the chloride ion.

(5) Michaelis-Menten Pharmacodynamic Test of Asc-2

A Michaelis-Menten pharmacodynamic test of a sodium-independent transporter Asc-2 transporting small-sized neutral amino acid was conducted. By investigating the change in the ratio of serine uptake by the difference in the substrate serine concentration, the Michaelis-Menten pharmacodynamic test of the Asc-2 was conducted.

The serine uptake test was conducted using the oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 with the 4F2hc or the rBAT (Asc-2-4F2hc or Asc-2-rBAT) was injected in accordance with the method in Section (2) in Example 2 described above. The results are shown in FIG. 11. The Km value of the serine transport of the Asc-2-rBAT was 2.88±0.37 μM (mean±standard error, n=3). The Km value of the serine transport of the Asc-2-4F2hc was 2.10 μM.

The Michaelis-Menten pharmacodynamic tests were conducted also for the amino acids, other than serine, which also serve as the substrates for the Asc-2, and the Km and Vmax values were calculated and are shown in the following Table 1.

TABLE 1 Km and Vmax values of substrate amino acids Km^(a) Amino acid μM Vmax^(b) L-Serine 2.88 ± 0.37 (1.00) L-Alanine 2.35 ± 0.30 0.83 ± 0.08 L-Threonine 2.72 ± 0.44 0.53 ± 0.06 L-Cysteine 3.13 ± 0.57 0.19 ± 0.03 Glycine 2.15 ± 0.35 1.18 ± 0.05 L-Valine 34.8 ± 10.6 0.79 ± 0.22 L-Leucine 39.6 ± 4.3  1.17 ± 0.11 ^(a,b)The Vmax value of each amino acid is shown by the ratio to the Vmax of serine. Each of the Km and Vmax values is represented by mean ± standard error (n = 3).

Each Vmax value in Table 1 is a ratio based on the Vmax of alanine being regarded as 1.00. Each values is represented by mean±standard error (n=3).

(6) Asc-2-mediated Amino Acid Release Test

In an oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 and with the rBAT (Asc-2-rBAT) had been injected together, the release of ¹⁴C-serine loaded as described above via the Asc-2 was investigated.

Into the oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 and the rBAT (Asc-2-rBAT) had been injected, 100 nl of 600 μM ¹⁴C-serine (˜10 nCi) was injected, and the cell was washed with an ice-cooled sodium-free uptake solution (Na⁺-free uptake solution) which does not contain serine, and then transferred into a sodium-free uptake solution (Na⁺-free uptake solution) in the presence or absence of serine (100 μM) at room temperature (18 to 22° C.), and then examined for the level of ¹⁴C-serine released from the cell.

The results are shown in FIG. 12. The Asc-2-rBAT exhibited a significant release of ¹⁴C-serine even in the absence of the extracellular serine, and the release was increased in the presence of the extracellular serine (FIG. 12). Accordingly, the Asc-2 is revealed to be a mixed transporter of the exchange transport and a facilitated diffusion type transport.

similarly to the Asc-2-rBAT, in an oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 with the 4F2hc (Asc-2-4F2hc) had been injected together, the release of ¹⁴C-serine loaded as described above via the Asc-2 was also investigated.

As a result, the Asc-2-4F2hc exhibited, similarly to the Asc-2-rBAT, a significant release of ¹⁴C-serine even in the absence of the extracellular serine, and the release was increased in the presence of the extracellular serine.

(7) Substrate Selectivity of Asc-2 (Inhibition Test Using Added Amino Acids and their Analogues)

In a serine uptake test in an oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 with the rBAT (Asc-2-rBAT) was injected, the effect of amino acids and their analogues on the system was investigated.

The serine uptake test was conducted using the oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 with the rBAT (Asc-2-rBAT) was injected in accordance with the method in Section (2) in Example 2 described above. Nevertheless, the test was conducted using as a sodium-free uptake solution (Na⁺-free uptake solution), and the ¹⁴C-serine (5 μM) uptake was measured in the presence or absence of various compounds (non-labeled) at 500 μM.

The results are shown in FIG. 13. Various neutral L-amino acid exhibited a cis-inhibitory effect. Especially glycine, alanine, serine, threonine and cysteine inhibited potently the uptake of the Asc-2-rBAT-mediated ¹⁴C-serine.

Acidic amino acids, basic amino acids, transport system L-specific inhibitor 2-amino-2-norbornane-carboxylic acid (BCH), γ-aminoisoyric acid and α-aminoisometylic acid exhibited no effect on the Asc-2-rBAT-mediated ¹⁴C-serine uptake (FIG. 13).

similarly to the Asc-2-rBAT, in a serine uptake test in an oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 with the 4F2hc (Asc-2-4F2hc) was injected together, the effect of amino acids and their analogues on the system was also investigated in the serine uptake test in the oocyte.

As a result, the Asc-2-4F2hc exhibited the behavior similar to that of the Asc-2-rBAT, and the part of the 4F2hc or the rBAT had no effect on the characteristics of the substrate-binding site of the fusion protein of the Asc-2 with the 4F2hc or rBAT (Asc-2-4F2hc or Asc-2-rBAT), and the data of the substrate selectivity obtained using the fusion proteins reflected the transport characteristics of the Asc-2 itself.

(8) Substrate Selectivity of Asc-2 (Uptake Test Using Various Amino Acids and their Analogues as Substrates)

Using various amino acids and their analogues as substrates, the uptake by an oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 with the rBAT (Asc-2-rBAT) had been injected was investigated.

The uptake test of various amino acids and their analogues was conducted using the oocyte into which a cRNA of the gene of the fusion protein of the Asc-2 with the rBAT (Asc-2-rBAT) had been injected in accordance with the method in Section (2) in Example 2 described above. Nevertheless, the test was conducted using as a substrate various compounds which were radiolabeled instead of ¹⁴C-serine.

The results are shown in FIG. 14. A substantial uptake into the oocyte was observed when using each of glycine (¹⁴C compound), L-alanine (¹⁴C compound), L-serine (¹⁴C compound) and L-threonine (¹⁴C compound) (all in FIG. 14) as a substrate.

Example 3 Immunohistochemical Analysis of Asc-2 Protein in Mouse Kidney

According to an ordinary method, a paraffin-embedded section of a mouse kidney was treated with an affinity-purified anti-Asc-2 antibody (1:100) and then stained with diaminobenzidine. In order to investigate the specificity of the staining, the treatment with the anti-AGT1 antiserum (1:100) in the presence of 200 μg/ml antigen peptide was also conducted.

As a result, an intense staining was observed in a collecting tubule in the area from the outer layer to the inner layer of a medulla in the mouse kidney as shown in FIG. 15 a. This staining was not observed when using the anti-Asc-2 antiserum in the presence of the antigen peptide, thus validating the specificity of the staining (FIG. 15 b). A microscopic observation at a further higher magnification revealed that the Asc-2 protein existed in the cortical collecting tubule (FIG. 15 c) and the collecting tubule of the outer layer of the medulla (FIG. 15 d) as well as in the luminal and basal membranes of the collecting tubule of the inner layer of the medulla (FIGS. 15 e and f).

INDUSTRIAL APPLICABILITY

An inventive sodium-independent transporter transporting small-sized neutral amino acid and its gene enables an in vitro investigation of the transport of the small-sized neutral amino acids and amino acid analogous including xenobiotics at the site where said transporter is expressed, and based on which, an in vitro assumption of the pharmacokinetics of these compounds is also enabled. Furthermore, the invention is useful in developing a pharmaceutical which permeates efficiently through a site where said transporter is expressed. Also by modulating an ability to transport a small-sized neutral amino acid and its analogue possessed by said transporter, the invention can be utilized in developing a method for controlling a cell proliferation. A method for analyzing a function of a transporter by constructing fused protein of the invention is a technology which enables the analysis of the function of a protein whose function can not be identified because of the inability to be transferred to a cell membrane since its cofactor required for the transfer to the cell membrane is unknown, and thus is useful in identifying the functions of various transporters whose functions have not been identified. 

1. An isolated protein consisting of the amino acid sequence represented by SEQ ID NO:
 1. 2. An isolated fusion protein consisting of the amino acid sequence represented by SEQ ID NO: 1 and a protein having the amino acid sequence represented by SEQ ID NO:
 3. 3. An isolated fusion protein consisting of the amino acid sequence represented by SEQ ID NO: 1 and a protein having the amino acid sequence represented by SEQ ID NO:
 5. 4. An isolated DNA encoding a protein consisting of the amino acid sequence represented by SEQ ID NO:
 1. 5. The isolated DNA according to claim 4 which consists of the base sequence represented by SEQ ID NO:
 2. 6. An isolated DNA encoding a fusion protein consisting of the amino acid sequence represented by SEQ ID NO: 1 and a protein having the amino acid sequence represented by SEQ ID NO:
 3. 7. An isolated DNA encoding a fusion protein consisting of the amino acid sequence represented by SEQ ID NO: 1 and a protein having the amino acid sequence represented by SEQ ID NO:
 5. 8. A vector comprising the DNA defined in any one of claims 4 to
 7. 9. The vector according to claim 8 which is an expression vector.
 10. A transformant which has been transformed with a vector according to claim
 8. 11. A method for identifying a substance which modulates an ability to transport a small-sized neutral amino acid in a sodium-independent manner possessed by a protein consisting of the amino acid sequence represented by SEQ ID NO: 1, which comprises a) providing a cell expressing the protein consisting of the amino acid sequence represented by SEQ ID NO: 1, b) incubating the substance and the small-sized neutral amino acid with the cell and c) determining an uptake of the small-sized neutral amino acid by the cell.
 12. An isolated nucleic acid comprising a partial sequence of consecutive 14 bases or more in the base sequence represented by SEQ ID NO: 2 or a sequence complementary thereto. 